TP-
Well, if you're telling her "I'm only looking to date my own age", you are ruling her out of contention for a relationship yourself... which feels a lot to her like you're rejecting her.
Same thing if you meet an 18-year-old, and she tells you "Oh, you're 27? I only want to date guys my age."
She might mean she wants you to take her to Pound Town, but it doesn't come across that way. Just seems like she's rejecting you.
A better way: she tells you she's 18 (or 24, or whatever it is), and you laugh and tell her "Oh my God, you're a baby!"
And then she asks how old you are. You tell her 27. You tell her you and her would never get along. You tell her you're not really into all that into boy bands anymore. Just tease her and make it sound like she's really young a little bit.
If she feels like you like her, but maybe aren't taking her totally seriously necessarily as a potential girlfriend, that can make it easier to remove the "boyfriend material" feather from your cap.
Chase